Sounds like you have 2 tables with the same information. There is nothing unusual about this because it's often how tables are related (e.g. PO_Number showing in tblPO as well as PO_Notes).
Not sure what you mean by
The issue that I am having is that the field is getting updated more than once.
If you mean more than one record is getting updated, that is one thing and should be described that way. The alternative is that the field in A record gets updated, then that field in that record gets updated again. Not sure how you'd know that unless you were stepping through code, which leads me to think it is the former.
Make a select query that returns a single record if that's what you want. Then turn that into an update query. Your issue is likely that more than one record satisfies the criteria, thus more than one gets updated. Try putting more criteria into it to restrict the records, or change the join type.
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